PTFE is a resin having good heat resistance and good chemical resistance. A porous PTFE film having a uniform and fine pore diameter can be obtained by stretching a film which is prepared by molding fluororesin particles containing PTFE as a main component into a film and sintering the molded film. Since such a porous PTFE film is composed of a PTFE material, the porous PTFE film has good heat resistance and good chemical resistance. In addition, the porous PTFE film has a uniform and fine pore diameter and easily has a high porosity. Therefore, such a porous PTFE film is used as a filter for removing fine impurity particles from a liquid. For example, PTL 1 describes a combination (hereinafter referred to as “laminate”) of such a porous PTFE film and a porous body functioning as a support layer that supports the porous PTFE film, and a method for producing the laminate. Such a laminate is suitably used in, for example, the field of semiconductor manufacturing as a filter for removing foreign substances from an etchant, a cleaning liquid, etc.
Recently, fine processing has advanced in the field of semiconductor manufacturing. With the progress of fine-processing technologies, the dimensions of foreign substances that are desired to be removed from an etchant, a cleaning liquid, etc. have also been decreasing. Accordingly, reduction in the filter pore diameter is desirable, but reduction in the filter pore diameter decreases the treatment flow rate. Reduction in the thickness of a porous PTFE film for the purpose of maintaining the treatment flow rate decreases the mechanical strength of the film.
In the case where the laminate of a porous PTFE film and a support layer that supports the porous PTFE film, the laminate being described in PTL 1, is used as a filter, in order to obtain a large filtration area, the laminate is often arranged in a filtering device as a filter having a pleated shape. However, there may be a problem in that, in folded portions of the pleats, leakage due to breakage of the porous PTFE film and expansion of the pore diameter of the porous PTFE film easily occur. Accordingly, development of technologies for reinforcing the laminate of a porous PTFE film and a support layer, the laminate being described in PTL 1, has been desired so that the laminate has good mechanical strength and can maintain heat resistance and chemical resistance, which are originally possessed by a PTFE film, while maintaining a fine pore diameter and a high treatment flow rate.
The reinforcement of a laminate of a porous PTFE film and a support layer that supports the porous PTFE film is achieved by a porous reinforcement layer. A widely used filter is a composite to which a mechanical strength is provided by laminating such a reinforcement layer on a porous PTFE film of a laminate including the porous PTFE film and a support layer. The reinforcement layer is a relatively thick film because the layer has a good mechanical strength, whereas the reinforcement layer is a porous film having a larger pore diameter than the porous PTFE film so as not to decrease the treatment flow rate of filtration.
Bonding between the reinforcement layer and the porous PTFE film is performed by, for example, a method in which the reinforcement layer and the porous PTFE film are simply laminated, and the resulting laminate is then fusion-bonded by heating to a melting point of PTFE or higher, or a method in which the reinforcement layer is bonded by using, as an adhesive, a tetrafluoroethylene-perfluoroalkyl vinyl ether copolymer (PFA), a tetrafluoroethylene-hexafluoropropylene copolymer (FEP), or the like. In the case where an adhesive is used, bonding is performed by providing a layer of a dispersion of PFA, FEP, or the like between the reinforcement layer and the porous PTFE film, and melting the PFA, FEP, or the like by heating. Accordingly, also in this case, heating to a temperature equal to or higher than a melting point of PFA, FEP, or the like is necessary for bonding.